The locker, labeled 'Giant' is a big, outdoor version of an Amazon delivery locker

Is Amazon getting ready to unleash a fleet of drones upon San Francisco, dropping Kindles from the sky? Or a self-driving car that can deliver the latest best-seller?

On Thursday, a huge orange locker was spotted in downtown San Francisco, in front of the city's iconic Ferry Building. On it were emblazoned the words, "Amazon Locker," along with the name, "Giant." The rectangular structure was large enough to hold a car, with most of one side of it taken up by what looked like a door. The locker also had an LCD screen and a keypad, along with a ticket scanner, built into its side.

Beyond that, it wasn't clear what the big box was for. Amazon did not immediately respond to questions. Neither did the San Francisco Recreation and Parks Department, which oversees Justin Herman Plaza, where the box was located. A worker at the scene suggested that the locker might be there for an event on Friday.

If the Giant locker is really moving in, either at that location or elsewhere in the city, it could represent the latest salvo in an escalating battle of delivery options. Other players, including Google and Uber, have been making moves in this area lately.

Amazon already operates its Amazon Locker service, which lets shoppers have their items shipped to an alternative location to their homes.

In the San Francisco Bay Area alone, Amazon has about two dozen of these locker units, typically smaller than the new behemoth, located inside stores such as 7-Eleven and Ace Hardware.

With the Giant locker, Amazon may be introducing a way for people to pick up their packages outdoors at any time of day. Or maybe it's the first of a network of larger lockers that could have delivery vehicles inside. It wouldn't be the craziest idea, given CEO Jeff Bezos' recent interest in aerial delivery drones.

Companies that aren't traditionally viewed as e-commerce players are becoming more interested in delivery services. Google now has its own delivery cars, through its Google Shopping Express service. Uber recently launched a "Rush" courier service.